1. Technical Field
The present invention relates to a disk drive unit, and more particularly to a method for re-ordering a command queue in writing or reading the data.
2. Description of the Related Art
A hard disk drive is one of the most popular external data storage devices for the computer. The hard disk drive has a magnetic head for reading the data stored in a magnetic disk, or writing the data into the magnetic disk. This magnetic head is mounted on an actuator mechanism that is driven by a VCM (Voice Coil Motor).
When reading or writing the data on the magnetic disk, the magnetic head is moved to a certain track and positioned by driving the actuator mechanism. The magnetic head is controlled to be moved to a certain position on the basis of the servo information stored in the magnetic disk.
The reading or writing of data is performed in accordance with a write command and a read command transferred from a host system. The hard disk drive re-orders a command queue as one of the performance improving techniques. That is, the command queue of write commands and read commands transferred sequentially from the host system is re-ordered so that the latency required to execute the writing or reading command may be minimized.
Various methods for re-ordering the command queue in the hard disk drive have been already offered. They are roughly classified into a method for improving the I/O throughput (amount of work performed over a period of time) and a method for preventing the time-out of command caused by re-ordering the command. And the method for improving the I/O throughput is further classified into a case where the seek time is longer than the rotational latency and a case where the seek time is almost equivalent to the rotational latency.
The method for improving the I/O throughput in the case where the seek time is longer than the rotational latency was employed in the hard disk drive or optical storage device in the past, and fundamentally involves selecting a command having the least seek time. Herein, the rotational latency is sufficiently shorter than the seek time, and thus ignored. Most of the methods use the seek distance instead of the seek time in relative length. The method called an SSTF (shortest Seek Time First) involves selecting a command having exactly the shortest seek time in the command queue. However, this method has a drawback that the seek distance is longer on the outer and inner circumference of the disk. The method for re-ordering the command queue using the cylinder number to resolve this drawback is called a SCAN method. These methods were described in Teorey and Pinkerton, “A comparative Analysis of Disk Scheduling Politics” , Comm. ACM. Vol. 15, No. 3, March 1972, pp. 177–184, Geist and Daniel, “A continuum of Disk Scheduling Algorithms”, ACM Trans. On Computer Systems, Vol. 5, No. 1, February 1987, pp. 77–92, and Denning, “Effects of Scheduling on File Memory Operations”, American Federation of Information Processing Societies (AFIPS) Joint Computer Conference Proceedings, Vol. 30, 1967, pp. 9–21.
Though the actual seek time and rotational latency should be considered, these methods use the seek distance and have the low degree of optimization.
As the seek time is shortened to the point that the seek time and the rotational latency may be equivalent in the time scale, the rotational latency can not be ignored. Thus, a method for reducing the rotational latency employing the cylinder number and the rotation angle (number) was disclosed in U.S. Pat. No. 5,570,332. Also, a method for re-ordering the commands in consideration of the seek time plus rotational latency to choose an executable command of the shortest time including the rotational latency was disclosed in U.S. Pat. No. 5,729,718. With this method, the latency (essentially, seek time+rotational latency) is calculated in consideration of the seek time, the rotational latency and the head change.
On the other hand, the methods for preventing the time-out of command were offered in Published Unexamined Patent Application No. 05-134810 (Fujitsu) and U.S. Pat. No. 6,145,052.
In using the hard disk drive, there is a request for taking preference of the power save and reduction in acoustic noise over its performance.
The conventional methods for the power save or reduction in acoustic noise involve JIT (Just In Time seek technology) by Seagate in United States, and delaying the seek time by an amount of rotational latency to make the power save and reduction in acoustic noise and enhance the performance consistently as disclosed in U.S. Pat. No. 4,899,234. Some methods involve directly the seek operation by switching the seek method (including the servo parameters) between a performance mode and a silent mode. These methods are very effective but required to have the parameters for the servo system duplicated, and therefore have a significant drawback of taking a great burden on the development because it must be checked that the seek is normally made for all combinations of the seek distance and the latency.